(1) Field of the Invention
The present invention relates to integrated circuits for a motor bridge interface, and particularly to control four external N-channel MOS power transistors in a H-bridge configuration for DC-motor driving.
(2) Description of the Prior Art
Motors play a key role in the increasing comfort and convenience in today's vehicles, providing functions from adjusting seats or headlamps to lifting windows and moving sunroof doors. However, making the motor easy to control, safe, and reliable isn't an easy task Most of the motors in today's vehicles conduct less than an average of 6 Ampere.
Motors normally need to be operated in both forward and reverse. This requirement leads to a circuit layout known as an H-bridge. FIG. 1 prior art shows a simple conceptual schematic of an H-bridge. A basic H-bridge has four switches, relays, transistors, or other means of completing a circuit to drive a motor. In FIG. 1 prior art the switches are labelled A1, A2, B1, and B2. Since each of the four switches can be either open or closed, there are 24=16 combinations of switch settings. Many are not useful and in fact, several should be avoided since they short out the supply current. For example, the motor spins forward if switches A1 and A2 are closed, the motor spins backwards if B1 and B2 are closed and the motor acts a brake if A1 and B1 are closed. The motor floats freely if all switches are open.
Very often power transistors are acting as semiconductor switches used in H-bridges. Semiconductor switches, as part of an H-bridge or an half-bridge, that are interconnected between a supply potential terminal, that carries a positive operating voltage, and a load output are characterized as high-side switches. Low-side switches, on the other hand, are interconnected between a load output and a second supply potential terminal, e.g. ground potential.
Very often power MOSFETs are used as digital switches. In a typical commercial application four MOSFETs were arranged in a H-bridge configuration, driving the motor forward or backwards. To switch a MOSFET on, the voltage at its gate must be some value greater than the supply voltage. To do this a charge pump is often used. A charge pump uses arrays of capacitors to increase voltage in a circuit. This higher voltage can be used to trigger the bases of the transistor arrays in an H-bridge. In this way, the voltage of the initial signal from the logic circuit needs not to be higher than that of the high-current load being driven.
A common way to control the velocity of a DC motor is through pulse width modulation (PWM). A motor is given full voltage and then turned off, cycling rapidly. Depending on the ratio between on and off time the motor will drive anywhere between full speed and stop. The frequency of this switching is generally above the bandwidth of mechanical switches.
The challenge for the designer of such motor controller is to find a reliable, efficient and cost effective solution. Generally transistors in bipolar or in Double Diffused MOS (DMOS) technology are being used. These technologies are expensive and it is desirable to find solutions that are less expensive.
There are some patents available for this area:
U.S. patent (U.S. Pat. No. 6,331,794 to Blanchard) describes a technique for supplying drive voltage to the gate of a high-side depletion-mode N-channel MOS-device for phase-leg circuits, H-bridges, or any circuit with a depletion-mode N-channel MOS-device with its source at a voltage above local ground.
U.S. patent (U.S. Pat. No. 6,185,118 to Sander et al.) discloses a driver circuit for driving a half bridge, that has a high-side semiconductor switch and a low-side semiconductor switch that are connected in series between a first and a second supply potential terminal, a drive is allocated to each of the two semiconductor switches that are respectively switched inhibited or transmissive by the respective semiconductor switches according to the direction of a drive signal. A load can be connected between the high-side semiconductor switch and the low-side semiconductor switch. For an inhibited state of a semiconductor switch, its drive terminal is charged approximately with the potential of the second supply potential terminal in order to attain a negative bias voltage of the drive terminal opposite the source terminal.
U.S. patent (U.S. Pat. No. 5,796,276 to Phillips et al.) describes a high-side gate driving circuit, where a current-mode differential error amplifier is used to regulate the current sourced to the gate. A current path is provided from the gate to the source of the power device, and a constant current is provided to the gate. In a single H-bridge, two transistors will be on at the same time; but the output capacitor of the charge pump lets it supply increased current transiently. A variable current source is also provided, and this current source is controlled by the output of the error amplifier. Preferably a voltage offset (avalanche breakdown diode) is interposed between the gate and source of the high-side driver; this ensures that the feedback loop will operate in a bistable mode, which avoids instability problems.